4.2: Digestion
By the end of this section, you should be able to:
- 4.1.1 Describe the digestive process.
- 4.1.2 Describe the organs of digestion.
- 4.1.3 Identify congenital anomalies within digestive organs.
The Digestive Process
The digestive process includes four steps: ingestion, digestion, absorption, and metabolism. Ingestion is the first step and involves the process of bringing food and liquids into the body through the mouth. Next is digestion , in which the body breaks down solid food through a process that includes both mechanical and chemical actions to form smaller, more absorbable substances for the gastrointestinal tract (Patricia & Dhamoon, 2022). Absorption is when cells take in the broken-down substances, and metabolism is when those absorbed substances are converted to energy.
Digestion occurs as an integrated process between the body’s upper and lower gastrointestinal tract and accessory organs. The body needs to have proper digestive processes to ensure correct function. Common end products of digestion are amino acids, simple carbohydrates, lipids, vitamins, and minerals.
Digestive Organs
The main organs of digestion, known as the gastrointestinal system , begin at the mouth and end at the anus. This system is divided into the upper gastrointestinal system , which starts at the mouth and ends at the duodenum (the start of the small intestine), and the lower gastrointestinal system , which starts at the small intestine and includes the large intestine (the colon, rectum, and anus). Other organs in the digestive system that aid in digestion but are not part of the main gastrointestinal system and don’t come in direct contact with food are the accessory organs: the pancreas , liver , and gallbladder (Figure 4.2).
The purpose of the digestive system , and all the systems that work in harmony with it, is to receive food, release nutrients and water from the food, and transport these nutrients to the bloodstream for energy production. The digestive system is also responsible for absorbing water from the colon and for excreting nondigestible solid waste from the anus (National Institute of Diabetes and Digestive and Kidney Diseases, 2022).
Upper Gastrointestinal System
The upper gastrointestinal system comprises the mouth, esophagus , stomach, and duodenum. The teeth, tongue, and salivary glands, which are all accessories for digestion, are in the mouth and facilitate the breakdown of food through mechanical (chewing) and chemical (enzymes) digestive process es. All of these components are required for the mouth to effectively complete its role in the digestive process.
After this initial breakdown, food then travels out of the mouth, through the upper esophageal sphincter, down the esophagus, through the lower esophageal sphincter, into the stomach , out of the stomach through the pyloric sphincter, and into the duodenum (Figure 4.3). The process of digestion through this upper gastrointestinal tract, starting at the mouth and ending in gastric emptying , can take 2–5 hours to complete (Readman, 2022).
Lower Gastrointestinal System
The lower gastrointestinal system comprises the small intestine, large intestine, rectum, and anus. Food entering the lower portion of the gastrointestinal system comes from the duodenum as chyme , a mixture of gastric juices and digested food. After digestive processes are complete in the duodenum, chyme moves through the small intestine, large intestine, and rectum, then out the body through the anus in the form of stool. Digestion through the lower gastrointestinal tract can take from 12 to 65 hours (Readman, 2022).
Accessory Organs and Systems
The pancreas, liver, and gallbladder are organs outside of the main gastrointestinal tract but are part of the digestive system (GI Associates and Endoscopy Center, 2022b). They directly aid in digestion by releasing enzymes and digestive fluids. Cholecystokinin , a hormone produced by the small intestine, triggers the pancreas and gallbladder to contract when fats and proteins enter. The pancreas, while also responsible for other body functions, produces amylase , lipase , trypsinogen , chymotrypsinogen , elastase , carboxypeptidase , nucleases , and protease . These enzymes break down sugars, fats, and starches (GI Associates and Endoscopy Center, 2022b). When sugar enters the bloodstream, the pancreas produces insulin , a hormone that assists in the absorption of glucose into cells to use for energy.
The liver produces bile , a digestive fluid that is stored in the gallbladder . When fatty foods enter the duodenum, the gallbladder releases bile to break down the fat into fatty acids . If the gallbladder is removed, the bile will instead travel directly from the liver to the small intestine.
Other accessory organs and tissues that assist in digestion include the salivary glands, tongue, nerves, and blood (University of Rochester Medical Center, 2022). When food enters the mouth and chewing begins, teeth break down the food into smaller parts, and saliva is released. Saliva is released by the sensory system, which is stimulated by sight, smell, and taste. Glands and mucosal membranes of the intestinal walls secrete small amounts of digestive enzymes and lubricating mucus to assist with waste movement and nutrient absorption.
Congenital Anomalies of the Gastrointestinal System
While rare, congenital anomalies can affect the digestive system. Most congenital anomalies result from undeveloped or incorrectly positioned organs, causing blockages that prevent food passage or the exit of waste products (Columbia University Irving Medical Center, n.d.).
Atresia, Stenosis, and Malformations
Congenital abnormalities include atresia , stenosis , and malformations. Some of these abnormalities can be detected in utero or soon after birth, while others remain undetected through childhood until they cause physical symptoms (Ludwig et al., 2022). Atresia is a condition in which a passage in the body is blocked. For example, esophageal atresia is when a baby’s esophagus has not developed correctly, and aural atresia is when the ear canal is underdeveloped. Stenosis, or narrowing, is a partial blockage of an area, and a malformation is an abnormal formation of a specific body part.
Anorectal anomalies can include an anus that is very narrow or totally enclosed by a layer of tissue or skin, a rectum that does not connect to the anus (imperforate anus), or a rectum that connects to a part of the urinary or reproductive tract through a fistula , an abnormal connection (Boston Children’s Hospital, 2022). Depending on which anomaly is present, the client may experience symptoms such as difficulty passing stool and constipation , total inability to have a bowel movement, or stool that exits through the urinary system or another system in the body, causing infection and incontinence (Boston Children’s Hospital, 2022).
Biliary anomalies affect the bile ducts. They can involve a progressive narrowing effect and/or a complete blockage, cystic formations that result in blockages in the biliary system, or biliary ducts that are malformed and do not properly connect to the pancreaticobiliary junction (Columbia University Irving Medical Center, n.d.; Stanford Medicine Children’s Health, 2023b). These anomalies fall into two main types, cystic and non-cystic. Cystic anomalies are caused by saclike structures filled with pus or other fluid that form and block different parts of the biliary system, whereas non-cystic anomalies are actual structural defects of the biliary system (Stanford Medicine Children’s Health, 2023). Common symptoms of these anomalies include jaundice , abdominal pain , pale or clay-colored stools , dark urine , enlarged liver margins, hyperamylasemia , enlarged splenic margins, and difficulty gaining weight.
Malrotation and Volvulus
Malrotation most often occurs in the small intestine and involves an incorrect rotation that happens during fetal development (Columbia University Irving Medical Center, n.d.). Volvulus is a severe complication of a malrotation in which blood flow is blocked to the digestive tract, causing ischemia , and/or to the intestines, causing stool blockage (Columbia University Irving Medical Center, n.d.). While malrotation can be the cause of volvulus , it can also occur in the absence of malrotation (Johns Hopkins Medicine, 2022).
Malrotations alone are usually asymptomatic. It is not until volvulus develops that a client experiences symptoms (Johns Hopkins Medicine, 2022) such as sudden and severe nausea and vomiting, dark red or bloody stools, severe constipation or difficulty expelling stools, abdominal distention , bilious vomiting (vomiting that contains bile), and shock .